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01 - Variations Under Domestication 01-01 - Causes of Variability 10 WHEN we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature.
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And if we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, we are driven to conclude that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species had been exposed under nature.

There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food.
Thomas Andrew Knight
Thomas Andrew Knight


It seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation; and that, when the organization has once begun to vary, it generally continues varying for many generations.

No case is on record of a variable organism ceasing to vary under cultivation.

Our oldest cultivated plants, such as wheat, still yield new varieties: our oldest, domesticated animals are still capable of rapid improvement or modification.

durum
durum

Angus
Angus
01 - Variations Under Domestication 01-02 - Effects of Habit 10 Changed habits produce an inherited effect, as in the period of the flowering of plants when transported from one climate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck;
duck
duck


and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents.
01 - Variations Under Domestication 01-03 - correlation of Growth 10 Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed.

I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably entail changes in the mature animal.

In monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St-Hilaire's great work on this subject.
Isidore Geoffroy Saint Hilaire
Isidore Geoffroy Saint Hilaire


Breeders believe that long limbs are almost always accompanied by an elongated head.

Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf;

but it has been lately stated by Mr. Tait that this is confined to the males.

Colour and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants.
From facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, whilst dark-coloured individuals escape:

sheep
sheep

pig
pig
01 - Variations Under Domestication 01-04 - Inheritance 10 The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and diversified.

It is well worth while carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, &c. and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other.
hyacinth
hyacinth

potato
potato

dahlia
dahlia


The whole organisation seems to have become plastic, and departs in a slight degree from that of the parental type.
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